Sputter coating machines, particularly those of the type identified above, are used in the manufacture of semiconductors. During a semiconductor manufacturing process, such machines sputter a thin layer, often of metal, onto a substrate, typically a semiconductor wafer of silicon or some other semiconductor material. The material that forms the deposited layer is supplied by ion bombardment of a target. Many types of targets are used in the same machines depending on the process to be performed and the semiconductor makers other requirements. Such targets are sometimes formed of a solid piece of sputtering material while others are formed of a mass of sputtering material bonded to a backplate. Targets of any of more than one size may usually be used in the same chamber of a sputtering machine. For example, in the machines referred to above, targets having nominal diameters of ten inches and twelve inches are available. While ten inch targets for these machines are typically 10.000+/−0.005 inches in diameter, nominal twelve inch targets are actually 11.625+/−0.005 inches in diameter. These targets are approximately one inch thick, which thickness declines over the life of the target as the sputtering material is consumed in the course of coating a large number of wafers, typically several thousand.
Sputtering targets of the vertical plenum machines referred to above are part of a sputtering cathode sub-assembly that mounts in an opening in the wall of a sputtering chamber in such machines. A sputtering surface of the target lies in the sputtering chamber and faces a semiconductor wafer that is to be coated with the material of which the sputtering surface of the target is formed. At the back of the target is fixed a magnet assembly that forms a magnetic field over the surface of the target to confine and enhance a plasma from which ions of gas for sputtering the target originate. In the machines referred to above, this magnet assembly typically includes an array of magnets that rotate behind the target. The provision of targets of two sizes (ten and twelve inch) and of two types (solid and backed) produces cathode assemblies for each of four distinct dimensional configurations for the same chamber of the machine.
The vertical plenum sputtering machines of the type identified above are high speed machines that cycle wafers through a series of processing stations by rotating wafer holders on an index plate in a vertical plane. The efficiency of such machines is in part due to indexing of the wafers in a fixed processing plane among the plurality of processing stations. Different processes, however, call for different target-to-wafer spacings. As a result, each of the four target configurations described above must be adapted to two or three different mounting positions in the chambers of the processing machine. This requires a variety of alternative adapter configurations so that any of the targets can be properly installed in the machine chamber for the performance of the various processes at the different target-to-wafer spacings.
In operation, the one of the cathode assemblies that is installed in a processing chamber of the machine has applied electrical power to it in the course of performing the sputtering process. This necessitates the insulating of the cathode assembly from the grounded wall of the machine. Annular insulators are used between the target or cathode assemblies and the machine housings, typically around the insides of the mounting adapters that hold the cathode assemblies in cathode mounting openings provided in one side of the chamber.
When the cathode assembly is installed in the processing chamber of a machine, the target surface faces a wafer holder for the semiconductor substrate across the chamber of the machine. During the sputtering process, electrical power and a negative charge are applied to the target, which causes gases in the chamber to become ionized to form a plasma and the negatively biased target to be bombarded with positive ions from the plasma. The bombarding causes atoms of the target material to be dislodged from the target, which then become mixed with the atoms of gases within the chamber. The object of the sputter coating process is to coat the wafer with the sputtered atoms of target material. However, the process also coats other surfaces facing the target as well as chamber component surfaces facing the gases within the chamber that scatter the sputtered atoms of material onto other surfaces. These surfaces include surfaces around the target and the mounting structure that holds the target in the opening in the chamber wall. Surface finishes and temperature gradients of surfaces in the chamber also affect the tendency for sputtered material to build up on these surfaces.
During the sputtering process, the material being sputtered builds up at specific areas on surfaces within the chamber. The buildup can cause spikes of coating material to form on these surfaces and create an icicle-like structure that can cause arcing and electrical shorts. The arcing can eventually cause the equipment to detect an electrical error condition and power down. This requires opening of the chamber and the possible loss of an expensive partially processed wafer. The buildup can also cause material to flake off of the parts during the sputtering process and to light upon and contaminate the semiconductor wafers being processed. Such contamination on the wafer surfaces can adversely affect the operation of the semiconductor devices or integrated circuits being made on the wafer. Such buildups require the stopping of the equipment and the cleaning of the chambers, resulting in a loss of productivity of expensive manufacturing assets.
One particular area where coating buildup can occur is on the surface of the insulator between the target or cathode assembly and the grounded mounting structure surrounding the cathode. Buildup of coating material, particularly where that material is metal, can cause a shorting of the power from the cathode to the grounded frame of the machine, which can cause damage and down time to the equipment and loss of wafers in the chambers of the machine.
FIG. 1 illustrates a target assembly 17 of an ECLIPSE type machine 10, discussed above, which includes a target 11 formed of a mass of sputter coating material and bonded to a metal backing plate 12. The backing plate 12 is secured to a rotating magnet assembly 13 to form an assembled cathode assembly 14. The cathode assembly 14 is mounted in a fixed size opening 15 in the door 16 of a chamber 18 of the processing apparatus 10, in which chamber 18 is a semiconductor wafer (not shown) mounted parallel to the target 11 for processing. An adapter ring 20, configured to support the specific target assembly 17 in the opening 15, is sealed to the chamber door 16 around its outer rim. The adapter 20 has an inner opening 21 in which the target assembly 17 is situated. An annular electrical insulator 22 is situated in the opening 21 and sealed to both the rim of the target assembly 17 and the adapter 20, as illustrated in FIG. 1A. A certain tolerance or spacing 23 around the insulator 22 is typical to accommodate the target assembly 17. The adapter 20 supports a grounded annular dark space shield 25 around the perimeter of the target 11.
During a sputter coating process, target material from the target 11 enters an annular space 26 between the rim of the target 11 and the adapter 20, and forms deposits 27 on the surfaces of the dark space shield 25 and the insulator 22. Plasma also can form in this space 26, which can sputter material from the edge of the target and re-sputter the deposited material 27 and redistribute it further into the space 26. A coating of conductive material 27 can eventually build up on these surfaces, including the surface of the insulator 22, thereby forming an electrically-conductive path 28 between the target assembly 14 and the grounded adapter ring 20, causing a current path through which power on the target assembly 14 can short to ground. This damaging occurrence has been avoided in this prior art structure by interrupting the operation of the machine 10 on one or more occasions during the life of a target and removing the adapter 20 and insulator 22 for replacement and cleaning. This interruption results in machine down-time and increases the cost of semiconductor wafers manufactured with the machine 10.
There is a need to improve the efficiency and reduce the costs of semiconductor wafer manufacture by solving the problems discussed above.